This invention relates generally to an electrostatographic printing machine, and more particularly concerns a control system employed to regulate the temperature of a fusing apparatus incorporated therein for affixing permanently a powder pattern, in image configuration, to a sheet of support material.
In the process of electrostatographic printing, electrostatic latent charge patterns are formed, and thereafter, rendered viewable. The field of electrostatography includes electrophotographic printing and electrographic printing. Electrophotographic printing is a class of electrostatographic printing which employs a photosensitive member to form, with the aid of radiation, a latent image. Electrographic printing utilizes an insulating member to form, without the aid of radiation, a latent image. In both, heat settable particles are generally employed. These toner particles are affixed permanently to the sheet by the application of heat thereto, i.e. fusing. Hereinafter, an electrophotographic printing machine will be discussed as an illustrative embodiment of the printing process.
In electrophotographic printing, a charged photoconductive member is exposed to a light image of an original document. The light image selectively dissipates the charge to record thereon an electrostatic latent image. A development system, thereupon, deposits toner particles onto the latent image rendering it visible. The toner powder image is transferred from the photoconductive surface to the sheet of support material. As hereinbefore indicated, the toner particles are generally made from fusable resins. When such toner particles are transferred to the sheet of support material, the powder image may be permanently affixed thereto by the application of heat. Heating the toner powder image partially dissolves the toner particles causing them to fuse into the sheet of support material.
Multi-color electrophotographic printing repeats the foregoing process a plurality of times for differently colored light image. Each development cycle deposits differently colored toner particles on the support material, in superimposed registration with the previously deposited layer of toner particles. In this way, the support material will have a multi-layered toner powder image deposited thereon. This multi-layered toner powder image is heated to coalesce. At this point in time it becomes transparent, i.e. each toner layer modulates the light rays passing therethrough, to form a copy having a single composite color. The modulated light rays transmitted through the toner layers to the eye of the observer. The observer thereupon sees only the resultant composite colors of the copy, i.e. those colors corresponding to the original document being reproduced.
In heating the toner powder image, it is preferable to raise the temperature of the support material so that it is substantially the same as the fusing temperature of the toner particles. In this manner, the support material functions as a heat source rather than a heat sink during the fusing operation. A suitable fusing apparatus may include a pair of heat sources, one to heat the support material and the other to heat the toner powder image. Various types of fusing systems have been developed which permit heating of the support material, as well as applying radiant heat to the toner particles disposed thereon. Exemplary of one such fusing apparatus for use in multi-color electrophotographic printing machine is described in U.S. Pat. No. 3,826,892 issued in 1974 to Draugelis et al. This fusing apparatus utilizes a radiant energy source and heated support material transport. The control system of the present invention will be described hereinafter, in conjunction with a fusing apparatus of this type. In operation, the radiant energy source and transport heat source operate in conjunction with one another to coalesce and affix the multi-layered toner powder image to the support material. The foregoing is achieved without charring or igniting the support material. Hence, the radiant energy source and transport heat source are controlled to insure that the temperature of the support material, with the multi-layered toner powder image deposited thereon, is appropriately regulated.
A control system hereinbefore utilized in conjunction with radiant heaters and transport heaters is described in U.S. Pat. No. 3,781,516 issued to Tsilibes et al. in 1973. As described therein, the fuser controller employs zero crossing and cycle stealing to regulate the power of the radiant heaters and support material transport heaters so that the proper amount of heat is available for fusing. However, a control system of this type requires approximately forty five minutes to one hour to set-up and adjust after assembly. In addition, it also somewhat difficult to maintain this fuser in an operating mode over the wide range of power levels required for multi-color fusing. Thus, this system may be expensive to manufacture and optimum reliability. Moreover, numerous active devices rather than passive devices are required for its implementation. Finally, the system utilizes a significant amount of power.
Accordingly, it is the primary object of the present invention to improve the control system regulating the heaters of a fusing apparatus arranged to affix permanently single or multi-layered toner powder images onto a sheet of support material.